Nitrile polymers are produced by polymerizing a major portion of a mono-unsaturated nitrile, such as acrylonitrile, and optionally a minor proportion of another mono-vinyl monomer component co-polymerizable with said nitrile in an aqueous medium in the presence of a preformed diene rubber which may be a homopolymer or a copolymer of a conjugated diene monomer.
Methods of preparing such polymers are known in the art, and details of such preparations can be found in U.S. Pat. Nos. 3,426,102, 3,586,737, and 3,821,348, all of these references being incorporated herein in their entirety.
The above processes produce a reaction product in which the latex solids, or resin, must then be recovered. It is desirable to recover such solids substantially free of moisture and contaminates. It is also essential that this recovery be as economical as possible with respect to time and cost of materials used.
Many techniques are known to recover resins. U.S. Pat. Nos. 3,248,455 and 3,345,430 describe methods of partially coagulating the latex into a paste, shaping the paste into strands, and passing these strands into a heated aqueous medium. Usually such treatment is followed by drying the resin to remove moisture. U.S. Pat. No. 3,821,348 discloses an alternate method to the above wherein coagulants are not used. Both of these systems, however, require many individual processing steps to obtain a quality resin.
A possible method to eliminate or simplify this processing is by using a screw devolatilizer as found in U.S. Pat. Nos. 3,742,093 and 3,799,234. As stated in these references, the volatilization of plastic material such as polyolefins in a screw extruder is well known.
Polymers obtained in this manner are thermoplastic materials which are easily processed and can be thermoformed into a wide variety of useful products. One problem that has arisen, however, is that certain stabilizers must be added to these resins to minimize color and thermal degradation, and to improve impact strength. Such stabilizers are extremely important with respect to rubber modified polymers. U.S. Pat. Nos. 3,907,937 and 3,997,628 describe such stabilizers. Other well known stabilizers have included Polygard, an organic phosphite stabilizer-antioxidant being tri-nonylphenyl phosphite produced by U.S. Rubber Company, and dilauryl thiodipropionate, (DLTDP) produced by Evans Chemetics Inc. The amount of stabilizers normally added to the latex is in the range of 2-4% based on the resin polymer.
It has been discovered that monounsaturated nitrile polymers containing a preformed diene rubber can be produced having good stability against loss of impact strength and resin yellowing when processed in a screw extruder with the use of stabilizers in the range of 0-1% of the resin polymer.